  
     ---0--- ---1---
    |\      |      /|
    | \     |     / |
    2  3    4    5  6
    |   \   |   /   |
    |    \  |  /    |
    |     \ | /     |
     ---7--- ---8---
    |     / | \     |
    |    /  |  \    |
    9  10  11  12  13
    |  /    |    \  |
    | /     |     \ |
    |/      |      \|
     ---14-- ---15--
  ||                  ||
  16                  17